1. Field of the Invention
The present invention relates to a method and an apparatus for processing FISU frames carrying no data according to the Signalling System 7 protocol and more particularly to a method and an apparatus for discarding or generating FISU frames without interrupting the processor.
2. Background Discussion
Many telecommunication products are operating according to the Signalling System Number 7 (SS7) protocol. This protocol defines four layers among the seven layers of the OSI (open system interconnect) standard. The four layers are the signalling data link called MTP-1, the signalling link functions called MTP-2, the signalling network functions called MTP-3 and the signalling connection control part called SCCP.
MTP 1 which is a physical layer, defines three types of frames. FIG. 1a shows the format of these SS7 frames also called Signalling Units (SU). MTP 2 uses these three types of SU: the Message Signalling Unit (MSU), the Link Status Signalling Unit (LSSU) and the Fill-In Signalling Unit (FISU).
The MSU, the LSSU and the FISU are respectively 278-byte, 7-byte and 5-byte long between flags. These three types of frames carry two specific data bytes BSN (backward sequence number) and FSN (forward sequence number) in common. The BSN and FSN bytes are respectively dedicated to the received and transmitted data frames and they are sequentially numbered. These numbers are modified according to the data traffic rate, and these bytes remain unchanged when no data are exchanged.
By referring to FIG. 1-a, the other initials stand for: PA1 F: flag PA1 LI: Length Indicator PA1 SF: Status Field PA1 SIO: Service Information Octet PA1 SIF: Signalling Informing Field PA1 CRC: Cyclic Redundancy Check PA1 (a) Receiving, in transmission mode, a FISU frame from the adapter and repeating FISU frames when no data are received from the adapter to transmit them to the network without interrupting the processor; and PA1 (b) Receiving, in reception mode, FISU frames from the network and discarding repeated FISU frames by transmitting idle state signals to the adapter without interrupting the processor.
The particularity of this protocol is: firstly, the format of the network link which is shown in FIG. 1b. Two consecutive SS7 frames are separated by one flag and the ending flag of frame (n) is the starting flag of frame (n+1). Secondly, when no data frame has to be exchanged which corresponds to idle state, the network links carry FISU frames.
In case of no traffic, these FISU frames do not carry any information, and this occurs frequently. On the adapter side, an interrupt is raised to a processor each time a FISU is received or transmitted that is to say every 6 byte. This environment is applicable to the low speed SS7 network in which the present invention is implemented. A low speed network is operating at a speed of 56 Kbps in the U.S. and 64 Kbps in Europe. Hence, an interrupt occurs every 857 microseconds or 750 microseconds in both transmit and receive directions. These too many interrupts degrade the performance of the SS7 adapter. Therefore it is necessary to process the FISU frames externally from the adapter by suppressing useless FISU frames received from the network and then these interrupts, and by generating FISU frames when idle states (no data signals) are transmitted from the adapter to the network without interrupting the processor.